Three American military trainers have been killed in an exchange of gunfire at the gates of al-Jafr Air Base in southern Jordan, the military said. The third wounded servicemen later succumbed to his injuries. It is yet not immediately clear what prompted the shooting.

The incident took place on November 4 at the gate of the King Feisal bin Abdul Aziz (al-Jafr) airbase in southern Jordan, when the car carrying the Americans did not heed the orders of the Jordanian guards to stop, the army says.

The shots were fired at a car carrying the trainers tried to enter the air base near the town of Mann. A Jordanian officer was also wounded

The U.S. Embassy in the Jordanian capital Amman said it had received reports about a security incident involving US personnel and that it was in touch with Jordanian authorities.

Pro-Western Jordan is a key member of the US-led military coalition against the extremist Islamic State group, which controls parts of neighboring Iraq and Syria.

Jordan also faces homegrown Islamic extremism, with hundreds of Jordanians fighting alongside Islamic State (ISIS) militants in Iraq & Syria and several thousand more supporting the extremist group in the kingdom.

Last November, a Jordanian police officer opened fire on a police training center staff during their lunch break at the cafeteria in Amman, Jordan, killing four, including two Americans, a South African, and a Jordanian. Six others were injured, including three Americans, a Lebanese, and two Jordanians, one of whom later died.

The gunman was then killed by a fellow Jordanian officer. The government subsequently portrayed the police captain as troubled.

The US has spent millions of dollars to help the kingdom fortify its borders. Last year, the US announced its intention to increase overall assistance to Jordan from $660m to $1bn annually for the 2015-2017 period.

For the West, any sign of instability in Jordan would be of great concern.